big: add random number generation
big: add random number generation

Adds func (z *Int) RandIntn(src rand.Source,n *Int) *Int

R=rsc
CC=golang-dev, gri
http://codereview.appspot.com/2315045

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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